We’ve seen that plants are actively defending themselves, but what about unprovoked attacks? Well, they’ve got that covered too!
Whilst plant invasion might not seem like revenge of the killer plant to us, to some plants it is literally a matter of life and death. As we touched on briefly when looking at animals, plant species have been moved all over the world and in doing so have endangered and killed other plants.
Today, 6,075 species of plants worldwide are documented as being invasive.
– State of the World’s Plants
How we talk about species both shapes our feelings and attitudes and reflects them. We talk about plants being alien to a setting, being foreign and being immigrants in the same way that we talk about people who have moved into a country.
The language we use suggests that these plants are active agents in their travelling, that they are intentionally invading. And on the whole, plants haven’t intentionally crossed borders. Yes some might blow over and seed but plants also get moved around the world accidentally on shoes, in freight containers and intentionally for display in gardens etc.
Plants have moved around the world and expanded their reach in the same way as humans have. And as with animals, this means competition and new threats to the native plants. As we saw with knapweed, it releases a chemical into the ground which literally kills other plants. It is this impact of a plant in its new habitat which seems to result in us talking about plants and animals which are somehow morally good or morally bad.
Alien species, not just plant ones, can have a huge economic cost in terms of management, environmental damage, introduction of diseases and can put local species at risk of extinction. This is particularly dramatic when you look at islands.
Islands tend to have a lot of specific species which have developed and adapted to that specific place, climate etc. For example 83% of Madagascar’s 11,138 occur nowhere else on earth. To introduce, accidentally or otherwise, new species can have catastrophic effects on that ecosystem. For example, common gorse, found originally in Britain, was introduced into New Zealand and thrived in the climate. However, it threatens to wipe out native plants and hence is routinely eradicated which costs time and money.
In a 2010 report, it was estimated that invasive non-native species cost the British economy at least £1.7 billion per year with plants as a group inflicting the highest costs to the economy. The top 20 non-native species inflicting the highest annual direct costs to the British economy included Japanese knotweed, rhododendrons, giant hogweed, Himalayan balsam and buddleia.
As well as impacting on biodivertisty and the economy, invasive plants also affect flooding. A number of riverside and aquatic invasive non-native plants are widely considered to increase the risk of flooding. They do this by clogging water courses with plant material or sometimes by causing riverbank eroding which leads to a build up of sediment.
Japanese Knotweed might well be the most well known of Britain’s invasive plants. It is a tall plant with bamboo like stems and was introduced in the early 19th century as a garden plant as well as use in stabilising railway banks. By the early 20th century, it was being regarded as a pest. This plant spreads rapidly and creates a dense mat of stems and roots.
It is widespread across Britain and causes damage to the urban environment. This includes damaging hard structures and surfaces, buildings and roads. It can crack tarmac, block drains, undermine foundations and invade homes. An article from 2017 says that it is estimated the Japanese knotweed costs the economy £166 million a year. It also contributes to river bank erosion and increases the likelihood of flooding.
You can even get an app which shows if your garden is at risk of invasion!
Rhododendrons are grown for their spectacular flowers and were first introduced in 1763. Whilst they were planted in gardens, they now affect most of Great Britain. Their crimes include blocking light, preventing other species from growing beneath them and leaving only trees that are able to grow above the level of the rhododendron canopy. It also carries diseases which are fatal to some native trees.
The impact rhododendrons have on the plant life of an area has an inevitable knock on effect on the animals which life there; habitats are destroyed and food sources depleted. For those creatures which turn to the rhododendron for sustenance, they find all of the plant contains toxins and animal deaths have been recorded following ingestion. This in turn removes a layer from the food chain and carnivores find themselves without prey.
Their impact even extends to waterways. Because of their height, they can grow on river banks but effectively cause the streams to become completely overgrown. This then reduces the insects etc which inhabit the area around the water, leaving invertebrate eating fish without food. I am assuming it also inhibits water plant growth which other species of water life eat.
Whilst the Rhododendron reeks havoc on the environment and ecosystems, giant hogweed reeks havoc on us.
It is, unsurprisingly, giant. It has massive leaves and can reach 20ft tall. Since being introduced to Britain, it has spread widely, preferring river banks but also growing in parks, cemeteries and wastelands. Whilst it was introduced as an impressive garden plant, it had escaped into the wild by 1828.
One of the main problems with the it, is that if the sap comes into contact with skin, in the presence of sunlight, it burns and blisters the skin. These injuries can last several months and leave the skin sensitive to light for many years.
So next time someone tries to tell you plants are passive and vulnerable, you’ll know better!